An undercover reporter has told a court he wanted to "expose criminality" about an alleged drug deal involving Tulisa Constostavlos and it was "outrageous" to suggest he spiked the singer's drink during one of their meetings.

Mazher Mahmood, dubbed the "fake sheikh", posed as a wealthy film producer called Samir Khan when he met the former X Factor judge at a string of luxury hotels and restaurants, jurors have heard.

Giving evidence at London's Southwark Crown Court, he denied "manipulating" the 26-year-old after he told her she was being considered for a leading role in a Slumdog Millionaire-type film tipped for Oscar glory alongside possible co-star Leonardo Di Caprio.

Contostavlos is on trial after allegedly boasting that she could "sort out" cocaine for Mr Mahmood and put the reporter in touch with her rapper friend Mike GLC who supplied the Class A drug.

She vehemently denies brokering the deal, which was exposed in the Sun on Sunday newspaper last June.

Appearing in the witness box behind a screen to protect his identity, Mr Mahmood said he used "subterfuge" when he secretly recorded meetings with the ex-N-Dubz star as he attempted to establish whether she was involved in drugs.

"The motive here is in the public interest to expose criminality," he said.

Under cross-examination, Mr Mahmood denied that the reason for the expose was to create a "sensational story".

He said: "Of course, I'm a journalist and stories are what I do for a living. That's not the reason I did it."

Mr Dein suggested that Mr Mahmood and his colleague "Nish" spiked Contostavlos's drink when they met at London's Metropolitan hotel on May 10 last year.

Mr Mahmood replied: "Absolutely outrageous."

Mr Dein said: "You got her to incriminate herself."

The witness replied: "I think that is nonsense."

Referring to Mr Mahmood's meeting with Contostavlos in Las Vegas in March last year, Mr Dein said: "This was only the beginning of your manipulation of Ms Contostavlos."

The journalist replied: "I disagree."

Sitting in the glass-walled dock, Contostavlos shook her head at times during Mr Mahmood's evidence.

Mike GLC, whose real name is Michael Coombs, 36, pleaded guilty on Monday to supplying half an ounce (13.9g) of cocaine - which tests later showed was at 37% purity - for £820.

Contostavlos, of Friern Barnet, north London, denies being concerned in brokering the drug deal.

Mr Dein asked Mr Mahmood whether his colleague Nish discussed a £3.5 million fee for the film role with Contostavlos.

Mr Mahmood replied: "I can't remember but it's possible."

He added: "I remember she asked for £3 million for this film part through her accountant on May 23."

Mr Dein said the film would tell the story of a female character from a "ghetto background" and that Mr Mahmood had given the impression that he wanted a "bad girl" for the part.

Mr Mahmood said: "The part in the film was a bad girl. I didn't want a bad girl to play it."

The jury was shown footage of the meeting in Las Vegas in which Contostavlos discussed the X Factor and told the undercover team she "wanted to get away from that".

Shortly afterwards, Nish - which is not her real name - pretended she was on the phone to Hollywood star Cameron Diaz, the court heard.

Mr Mahmood, who was accused by Mr Dein of "lying to the jury" during parts of his evidence, denied that it was later suggested to Contostavlos that she may lose the part to Keira Knightley.

The court has heard that Contostavlos spoke about being able to get "white sweets" and "green sweets" - allegedly code for cocaine and cannabis - while at the Metropolitan Hotel with Mr Mahmood on May 10 last year.

The singer was also said to have bragged that she used to be part of a gang who sold crack cocaine and that her ex-boyfriend was a cocaine dealer.

Mr Dein said: "The plan from the beginning was to get Ms Contostavlos very heavily under the influence of alcohol."

Mr Mahmood said he discussed Contostavlos's fee for the film with her PA Gareth Varey at the Metropolitan hotel on May 10 last year.

Mr Varey said the singer was expecting £2 million for the next series of X Factor after previously being paid £1.5 million and she would want £3 million for the movie, the witness said.

The undercover reporter denied that his colleague Nish had spoken earlier to Contostavlos about the film deal in the toilets of Nobu restaurant, the popular celebrity hangout in London, without her recording device.

Mr Mahmood said: "She's a celebrity. You don't have a conversation like that in a toilet in a restaurant like Nobu."

The court was played an audio recording from May 10 in which Contostavlos was allegedly heard telling Mr Mahmood her ex-boyfriend is a cocaine dealer.

Mr Dein asked the witness whether he touched the singer on the thigh "on one or more occasion" to "create a friendly and inviting" atmosphere during the recorded conversation.

Mr Mahmood replied: "No, not true."

He told the court that Mr Varey was "visibly drunk" at the end of the night but denied that Contostavlos was in the same state.

Contostavlos started giggling in the dock and put her hand over her mouth as a video was played of the pair returning to Mr Mahmood's hotel suite shortly after 2am.

In the recording, Mr Varey could be heard discussing Contostavlos's fee for the film role with Mr Mahmood and said £2 million was "on the table" for the next X Factor series.

Mr Mahmood denied that he deleted one of his recordings or influenced the statement of a driver, referred to as Mr Smith, who told police he heard Contostavlos speaking out against drugs.

He said: "If I had he would be straight on to the Guardian newspaper."

Mr Mahmood said his undercover operation was "absolutely about the story" but denied he would have gone to "any lengths".

He added: "If she had not supplied cocaine there would be no story."

During re-examination, prosecutor Tim Cray asked Mr Mahmood if there had been anything to suggest Contostavlos was "play acting" during her conversations about drugs. He replied: "Nothing whatsoever."

In the video recorded in Mr Mahmood's Metropolitan hotel suite, which was played to the court, the reporter was heard asking Mr Varey if X Factor judge Simon Cowell was gay.

Mr Varey replied: "Yes."

Mr Mahmood asked: "Have you slept with him?"

Mr Varey replied: "No."

Asked why, he answered: "I will when I make my move."

Mr Mahmood said: "How do you know he's gay?"

Mr Varey replied: "I know people who have."

The jury was sent home and told to return to the court on Monday.

A spokesman for Simon Cowell said: "We are bound by contempt of court rules so cannot respond to these claims in the manner we wish to.

"We are speaking to the Attorney General's office - and we intend to make a statement in court as soon as possible."